This invention relates to an improved process for producing the Freon(copyright) clathrate of beclomethasone dipropionate.
Beclomethasone dipropionate is a commercially important corticosteroid used for the treatment of asthma and allergic rhinitis. It is often administered in the form of an inhaler containing beclomethasone dipropionate as a trichloromonofluoromethane (Freon(copyright) 11) clathrate as described in U.S. Pat. Nos. 4,044,126, 4,414,209 and 4,364,923.
Processes for preparing the Freon(copyright) 11 clathrate of beclomethasone dipropionate are described in U.S. Pat. Nos. 154,044,126, 4,414,209 and 4,364,923. The currently preferred process involves adding a solution of beclomethasone dipropionate in hot methanol to a large volume of Freon(copyright) 11. The methanol is then removed by azeotropic distillation.
The current process is inefficient, requiring the use of large volumes (e.g. 218 kg) of Freon(copyright) 11 per kg of beclomethasone diproprionate because the methanol/Freon(copyright) 11 azeotrope only contains about 2% methanol. The use of large volumes of Freon(copyright) 11 is undesirable due to the rapidly escalating cost of such chlorofluorocarbons. In addition, there are significant environmental concerns surrounding the use of Freon(copyright). The development of a process which would not require large volumes of Freon(copyright) 11 is therefore highly desirable.
The ability of certain molecular sieves to bind methanol is known in the art. The use of molecular sieves to remove a solute which is a small polar molecule, such as water or methanol, from a solution is described in U.S. Pat. No. 4,864,012. The use of molecular sieves to remove water from Freon(copyright) refrigerants is also known.
Silica gel is a colloidal form of silica known in the art as a dessicant, and as an adsorbent used as the stationary phase in chromatographic separations.
The present invention provides a process for preparing beclomethasone dipropionate Freon(copyright) 11 clathrate using low volumes of Freon(copyright) 11.
The present invention provides a process for forming the beclomethasone dipropionate Freon(copyright) 11 clathrate comprising the use of an agent which selectively binds methanol to selectively remove methanol from a mixture of methanol and Freon(copyright) 11 containing beclomethasone dipropionate.
More particularly, the present invention comprises combining a solution of beclomethasone dipropionate in hot methanol with Freon(copyright) 11, heating the mixture to reflux temperature to generate a vapor, contacting the vapor with an agent which selectively binds methanol, condensing the vapor and returning the condensed vapor to the mixture. Alternatively, the present invention comprises combining a solution of beclomethasone dipropionate in hot methanol with Freon(copyright) 11, heating the mixture to reflux temperature to generate a vapor, condensing the vapor, contacting the condensed vapor with an agent which selectively binds methanol, and returning the condensed vapor to the mixture.
Preferably the methanol binding agent is molecular sieves, and more preferably type 3A, type 4A or type 5A sieves.
In a particularly preferred embodiment, the instant process utilizes about 4 L of methanol, about 7.5 L of Freon(copyright) 11 and about 32 kg of molecular sieves per kg of beclomethasone dipropionate.
After use the molecular sieves can be regenerated, by heating to 1500xc2x0 to 200xc2x0 C. to desorb the bound methanol. Regenerated sieves can be subsequently reused in the process of the present invention.
The Freon(copyright) 11 can also be distilled and reused in the process of the present invention. In addition, the process of the present invention can be carried out in an essentially closed system thereby preventing the release of Freon(copyright) to the environment.
As used herein the term xe2x80x9cmethanol binding agentxe2x80x9d means a solid reagent that selectively and irreversibly binds the methanol in a methanol/Freon(copyright) mixture. A preferred methanol binding agent is molecular sieves, and more preferably type 3A, type 4A or type 5A sieves. Another preferred methanol binding agent is silica gel. xe2x80x9cIrreversible bindingxe2x80x9d means that the methanol remains bound to the binding agent until subjected to suitable conditions to effect its release. In the case of molecular sieves, methanol remains bound to the sieves until subjected to heating to high temperature, e.g. 1500xc2x0 to 2000xc2x0 C.
The use of methanol to dissolve the beclomethasone dipropionate is necessary due to the relative insolubility of the steroid in Freon(copyright) 11. The beclomethasone dipropionate remains at least partially soluble in the mixture formed by combination of the methanol solution with the Freon(copyright) 11. Removal of the methanol solvent from the beclomethasone dipropionate/methanol/Freon(copyright) 11 mixture serves to drive the crystallization of beclomethasone dipropionate from solution in the form of the Freon(copyright) 11 clathrate.
To carry out the process of the present invention, dissolve beclomethasone dipropionate in a minimal amount of hot methanol, preferably utilizing about 4 mL methanol per gram of steroid. Add the hot solution to a suitable volume of chilled Freon(copyright) 11, preferably 7 mL to 8 mL of Freon(copyright) per gram of steroid, and most preferably about 7.5 mL of Freon(copyright) per gram. Heat the stirred mixture to reflux temperature to generate a vapor which is a Freon(copyright) 11 methanol azeotrope. Contact the vapor with a methanol binding agent, preferably silica gel or molecular sieves, more preferably molecular seives, and most preferably type 3A, type 4A or type 5A sieves, then condense the vapor and return the condensed vapor to the mixture. Reflux the mixture for about 4 hours or until substantially all of the methanol has been removed (e.g. until less than 2% methanol remains in the mixture). Cool the mixture and filter to collect the solid Freon(copyright) clathrate.
In an alternative embodiment, generate the vapor as described above, condense the vapor, contact the condensed vapor with a methanol binding agent, preferably silica gel or molecular sieves, more preferably molecular sieves, and most preferably type 3A, type 4A or type 5A sieves, then return the condensed vapor to the mixture. Continue refluxing until substantially all of the methanol is removed, as described above, and collect the Freon(copyright) clathrate by filtration.